Contacting probes with both ends of a printed pattern is one of the methods for inspecting the continuity of the pattern on a printed circuit board. Continuity of the printed pattern can be inspected by checking whether a predetermined current flows when a voltage is applied between the probes.
This method, however, is difficult to apply to a pad section with a fine pitch in which the ends of adjacent printed patterns are very close to each other. That is, probes compatible with fine pitches are generally expensive and a technique for aligning probes with a substrate very accurately must be used during inspection. In addition, since the probes contact the pads, the latter may be damaged.
One inspection method which attempts to solve this problem uses anisotropic conducting rubber instead of the probes, Japanese Patent Application Laid Open No. 61-62877. This method enables the continuity of a printed pattern with a fine-pitch pad section to be inspected without using expensive probes compatible with the fine pitches and without damaging the pads. With the method using anisotropic conducting rubber as illustrated in FIG. 14, however, pressing the rubber 2 against a circuit board 8 may not result in the bottom surface 2a of the rubber 2 abutting the top surface 4a of pad 4 if there is a resist 6 having a top surface 6a significantly above the top surface 4a of the pad 4. This of course results in poor contact during inspection.
Other methods attempting to solve this latter problem use a non-contact sensor, Japanese Patent Applications Laid Open Nos. 4-244976 an 58-38874. This method enables a signal to be transmitted between the pads and the sensor despite the resists around the pads.
Since, however, the non-contact sensor uses a sine-wave alternating current and must include filtering or include a wave detector to reduce the effects of noise, several tens of milliseconds are required to inspect a single printed pattern. Thus, if several thousand inspections are carried out for a single circuit board, several seconds are required to inspect the circuit board.
Another method for measuring a direct transient current in a non-contact manner to determine the acceptability of a printed pattern and which does not use a sine-wave alternating current is disclosed in Japanese Patent Application Laid Open No. 3-154879. This method eliminates the need for a wave detector, thereby somewhat reducing the inspection time.
As shown in FIG. 15A, the method connects terminals 20a and 20b of an inspection apparatus 20, a printed pattern 22, and a reference potential surface 24 formed to ensure insulation from the printed pattern 22, respectively, and applies a direct current to the terminal 20a for inspection. That is, the method determines the acceptability of the printed pattern 22 based on a transient current flowing through a capacitor formed between the printed pattern 22 and the reference potential surface 24. If, however, the printed pattern 22 is open-circuited at a position remote from the terminal 20a, as shown in FIG. 15A, the method may fail to detect the open circuit. In addition, if both printed patterns 22a and 22b are open-circuited and short-circuited, the method may also fail to detect this condition.
In addition to the forgoing and due to the need to examine the capacity of the capacitor between the printed pattern 22 and the reference potential surface 24 as well as a steady-state current flowing between the terminals 20a and 20b, the method must wait for a capacitor with a relatively large capacity to be almost fully charged. This of course prevents significant reduction in inspection time.
It is a general object of the present invention to provide an inexpensive and reliable circuit board inspection apparatus and method that can be applied to a circuit board with high density wiring.